The ultrastructure of dense bodies of various smooth muscles continues to be studied in the electron microscope. The protein constituents of dense bodies, particularly those comprising the 10 nm filaments are being studied by SDS gel electrophoresis, peptide mapping and immunological techniques using electron microscopy to monitor protein purification. The protein mesosin, a major component of 10 nm filaments of smooth muscle, is also being searched for in a variety of other cell types including axons.